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Active Member

Hey there I'm machining my fist steel on my mini lathe, and I can't seem to get a smooth finish.
I've tightened everything the most I can, I'm ground my own HSS tools, to the best of my ability so far I'm running the lathe at about 500~ RPM's.
I'm attaching some photos.
Any advice is VERY appricated.

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Active Member

From photo of the part, it looks like the tool bit might be rubbing or isn't sharp. It's hard to tell from the tool bit photos, but you should have a good sharp cutting edge, and make sure the toolbit is not above center of the workpiece. If it is, the area below the cutting edge will be rubbing on the workpiece. Is it mild steel that you are cutting?

Active User

If your file will actually cut the tool bit then the tool bit is not hard enough to keep an edge.
I've found that a very sharp edge honed with a simple diamond tool/plastic stick helps immensely. I use these EZE-LAP-3pc-Diamond-Hones
Hot rolled mild steel is usually horrible to turnhttp://www.conradhoffman.com/advancedsharp.htm and try out the sheartool, My goto tool bit on a cantankerous part.
When facing off some hot rolled plate I wound up the lathe to its fastest speed and used a braised on carbide tip that I diamond honed to a sharp edge. My HSS bits just wore away from the weld and heat scale.
start at post 114 to see the start and finish of how smooth it became https://www.hobby-machinist.com/threads/the-giant-binocular.55688/page-4#post-618084
a word of warning, the chip peels off in one continuous fast razor sharp and blue heated chip so keep well clear.

Active Member

I'm a brand new newbie as well and I was having similar difficulties yesterday so I'm following this thread with interest.

I wondered about the quality of the random steel offcut* I was playing with. When I filed it--500 rpm or so, first with an 8 inch and then a 6 inch file--it filled up several of the teeth on the files. Is that an indication of difficult-to-machine steel? BTW, the surface after filing still wasn't that good. I wasn't sure if I needed to remove more of the prior rough cut surface or if the file was also producing a poor finish. Ran out of time before achieving an decent results.

Craig
* box lot from an auction including a bunch of brass and aluminum flat and round stock.

Active Member

I fu**King cracked it!!!
OK here's what I did first ensure the tool is a smidge below center like 0.1mm then I sharpened my tools again increased the angle on the front so it was Not tearing now that improved it a bit but what really killed it was dousing it in oil whilst actually turning it and reducing the speed down to more slower than seemed right. After that the cut sounds different if that makes sense...

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John York

You cannot expect to get a smooth finish when contouring an irregularly shaped cut such as you show, a pointed tool will remove metal in the quickest fashion, but to achieve a decent finish on a contour, you need to use a round nosed tool with back and side rake, moving both feed directions slowly, also use of a cutting oil is necessary to avoid tearing. To test tools and procedures for finish cutting, you should be taking straight cuts, not contours. In our high school machine shop, we used a round nose tool, taking light cuts for finishing, using cutting oil for lubrication, then using (possibly) a file, then emery cloth.
If, when filing a part on the lathe, to avoid pinning of the file teeth, one can rub chalk onto the face of the file, and frequent cleaning of the file teeth is necessary to avoid loading up of the teeth and subsequent pinning; use a file card ( a flat brush with short stiff teeth on one side and bristle brush on the other side) every machine shop should have one, not just for lathe filing, but for keeping all your files clean so that they do not gouge up any and all filing work.

Flood coolant is almost essential if a nice surface finish is required.
This is 1045 CR turned at 400 SFM (6 1/2" diameter at 250 Rpms) .020" /.5MM doc and a feed rate of .008" /.200MM per revolution using a common triangular carbide insert tool with a 1/64" /.25MM nose radius.
The surface finish was excellent considering the material

H-M Supporter - Gold Member

Been a lot of discussion on this topic lately . A easy was to take the guesswork out of being on center ........... Take a piece of round stock and drill a 1/4" hole thru it , drill and tap 2 holes in it and put a 1/4" rod thru it . Set up a turning tool and find dead nuts center taking face cuts and adjusting . When you're there , set that little piece you just made on your tool tip and set it to a piece of flat stock across your ways or carriage etc . Lock it up and keep it on the lathe forever or until you can afford a height gage . It's an easy tooling aid .

Active Member

Using Emery, Files, is not cheating. Any method that achieves the result you want, is the correct method for you. Emery cloth is not made, just to sell to the likes of us,. not enough business. It is made for "Professional Engineers" who use it all the time.
You will find, that some ALL METALS, Brasses, steel, aluminium, are like women. Some are easy to manipulate, ie, turn or polish, others will reject every advance you make. Turning copper is something else again.
Sharp tool bits, is the key. Even burnish the cutting edge of your bit, on some emery paper. Back the paper with something flat and rigid, and rub the tool face carefully against it
Your wife or girlfriend has some handy tools also. Their finger nail polish board, does a nice job honing HSS edges. Nail polish, Lacquer, makes an excellent coating for electronic circuit boards, waterproofing! Scissors cut tin plate, flour sieves are good for moulding sand, and their knickers feel nice worn under your strides.
Kidding, sort of, about the last bit, but the other stuff is f'real.

H-M Supporter - Sustaining Member

Been a lot of discussion on this topic lately . A easy was to take the guesswork out of being on center ........... Take a piece of round stock and drill a 1/4" hole thru it , drill and tap 2 holes in it and put a 1/4" rod thru it . Set up a turning tool and find dead nuts center taking face cuts and adjusting . When you're there , set that little piece you just made on your tool tip and set it to a piece of flat stock across your ways or carriage etc . Lock it up and keep it on the lathe forever or until you can afford a height gage . It's an easy tooling aid .

You worry entirely to much about tool height, I set the tool in the video with an 18" steel ruler, on this lathe the spindle is 12 7/16" above the ways.
The ONLY time that I find it important to get the tool on center within .005" is when parting to 0 in the X axis.

This is a smaller lathe that is 2axis CNC (I should say that it is 2.5 axis because it will thread, it will also operate all 3 axes when taper threading) the C axis is not under full control however.
I set the turning tools with a ruler and the parting tools by trial.

H-M Supporter - Gold Member

Not me , I don't worry about it at all ! Just giving free advise to someone asking to learn . I preset all my tools and never have to check them . Some people aren't fortunate enough to be able to do this , or work with a scale or obtain a height gage . This is a cheap alternative to all .

Not me , I don't worry about it at all ! Just giving free advise to someone asking to learn . I preset all my tools and never have to check them . Some people aren't fortunate enough to be able to do this , or work with a scale or obtain a height gage . This is a cheap alternative to all .

You worry entirely to much about tool height, I set the tool in the video with an 18" steel ruler, on this lathe the spindle is 12 7/16" above the ways.
The ONLY time that I find it important to get the tool on center within .005" is when parting to 0 in the X axis.

This is a smaller lathe that is 2axis CNC (I should say that it is 2.5 axis because it will thread, it will also operate all 3 axes when taper threading) the C axis is not under full control however.
I set the turning tools with a ruler and the parting tools by trial.

Active Member

As T Bredehoftsaid there is some mild steel that is 'ropey' and extremley difficult to turn a good finish on. Good that you are playing around to see what works for you on your machine. Here's a couple of YouTube vidioes that may be of help:

Active Member

Been a lot of discussion on this topic lately . A easy was to take the guesswork out of being on center ........... Take a piece of round stock and drill a 1/4" hole thru it , drill and tap 2 holes in it and put a 1/4" rod thru it . Set up a turning tool and find dead nuts center taking face cuts and adjusting . When you're there , set that little piece you just made on your tool tip and set it to a piece of flat stock across your ways or carriage etc . Lock it up and keep it on the lathe forever or until you can afford a height gage . It's an easy tooling aid .

Active Member

To everyone else thank you SO much for enaging, I'd watched those vids before @NortonDommi,
"Their finger nail polish board, does a nice job honing HSS edges" this advice to gold now I don't have to buy anything I can just steal

H-M Supporter - Gold Member

Sure will . I'm at work now though . Don't let anyone tell you that centering your tools isn't important . Yes , for junk work it may not matter much , but you're going to chew inserts and carbide to shreds if not on center or reasonably close . Give or take a few thou . I'm turning and facing 15-5 SS at 32 Rockwell C at the moment , and it don't like to cut without taking a good bite out of it . I'de be busting cemented carbide tools to bits if I wasn't on center .

As far as the tool aid , think of an in-movable depth gage . Once you establish the CL of your lathe , it never changes , so why anyone would just estimate with a scale is beyond me . Also , as you progress in the trade and use form tools , if you aren't on center , you can't get true form of the tool . This is advanced for some , but if you're working with engineers that need their part perfect , it matters . Learn the good traits of machining early , they'll save you aggrevation years down the road .

By the way , spent 2 weeks over in Ireland last years this time . Boy got his masters at U of Limerick . I would live there in a heartbeat !

Brass

The easy way ! Put a center in the spindle taper. Move the saddle up to the chuck. Put a block of material on the top slide and use the center to scribe a line on the block. Job done ! Simply adjust your tool hight to match the scribed line.

I have an "L" shaped piece of scrap alloy that I just sit on the top slide with a line scribed across it. Works every time

Active User

Vimes, I'd like to offer an opinion. I think the main cause of your issues is your tool. The pics you posted earlier give us some hints. The tool on the left has side and back rake, which is good. We cannot clearly see the relief angles but they look too small to my eye. The nose radius is too big so the tool will deflect big time. The tool on the right appears to have the rake angles inclined in the negative direction, and that tool's nose radius is also too big; this tool will not cut well. It will help to show us better pics of the tool.

Your tools need to be sharp. Buy some diamond stones and hone the edges. Working with dull tools doesn't work well.

I'm in the "get the tool on center" camp. For HSS tools, I feel it is important to get the tool dead on center, not above or even a little below. Go below and the work wants to climb onto the work. You might think this is just hobby guy BS but if you ever work on small diameter stuff to close tolerances you will understand why this matters. Make yourself a height gauge that is set to the exact centerline of your lathe spindle and use it every time you mount a tool.

If you can, find some leaded steel or aluminum to learn with. Using mystery steel or worse, stuff like 1018, will only frustrate you.

Finally, keep in mind that you're using a mini-lathe with very limited power, rigidity and speed. HSS is the best tooling choice for these little lathes. I suggest you spend some time learning to grind good ones.